We are presently exploring the use of intra-operative radiation therapy as a method of treating patients with abdominal and retroperitoneal tumors which have a low cure rate by conventional means. Although we have some information from previous clinical data on the tolerance of normal retroperitoneal structures to doses of intra-operative radiation therapy of approximately 2000 rads, full information is not available as to effects at high dose levels and the long term toxicity which may be expected. We have therefore proceeded on a series of experiments to evaluate the effect of intra-operative irradiation using canines as the experimental model. Dogs receive an exploratory laparotomy and then receive high dose radiation therapy to normal retroperitoneal structures. Toxicity has been evaluated at times up to 2 years after irradiation. Present results indicate that doses up to 3000 rads are acceptable with minimal toxicity but doses of 4500 rads may produce unacceptable toxicity. Toxicity has been specifically noted in intestines, ureter and kidney. We have not noted significant problems in large abdominal blood vessels or in the retroperitoneal soft tissue.